The principal source of natural rubber is the rubber tree Hevea brasiliensis. Relatively recently, there has been renewed interest in the development of guayule rubber which can be recovered from the shrub Parthenium argentatum by several processes (Guayule, An Alternative Source of Natural Rubber; National Academy of Science; Washington, D.C.; 1977). The guayule shrub grows in semiarid regions not really suitable for food crops; therefore, the guayule shrub could utilize marginal land to produce a natural rubber very similar to tree rubber (Hevea rubber).
Unlike the Hevea tree which can produce rubber latex for many years, the guayule shrub must be harvested and processed to recover rubber. The basic reason for this requirement is that rubber latex forms in ducts in the Hevea tree, whereas rubber latex in the guayule shrub is contained in individual cells (ibid, page 24) which must be ruptured to release the rubber. Generally, the whole guayule shrub is harvested and processed to recover the rubber. An alternative procedure is to cut the portion of the shrub above ground and process the top portion of the shrub allowing the root system to generate a new plant. This procedure is referred to as "pollarding".
Regardless as to whether whole or pollarded shrub is processed, economics dictate that preferably the contained rubber as well as non-rubber constituents of the guayule shrub be utilized to improve the commercial feasibility of a guayule rubber industry. A major component of the guayule shrub is resin. The resin content of a typical shrub will range from 5 to 15 percent (ibid, page 35).
Guayule resin can be extracted from resinous guayule rubber (ibid, page 39) or can be extracted directly from the shrub prior to recovering rubber (Paper No. 54, "Guayule Shrub Deresination", presented at the 117th Meeting of the Rubber Division, ACS, Las Vegas, Nev.; May 20-23, 1980).
Unlike Hevea rubber, guayule rubber does not contain natural anti-oxidants and must be stabilized to prevent rapid degradation upon contact with air (ibid, page 25). Since guayule resin is very soluble in guayule rubber, it may be concluded that guayule resin does not or will not function as a stabilizer against the air oxidation of guayule rubber.
Winkler and Stephens have reported that guayule resin functioned as a peptizing agent for natural Hevea rubber and styrene-butadiene rubber (SBR). Specifically, the authors found that guayule resin promotes effective reduction in the gel content of Hevea and SBR (D. S. Winkler and H. L. Stephens, "Plastification Effect of Guayule Resin on Raw Rubber"; presented at the 2nd International Guayule Conference held in Saltillo, Mexico in 1977). The authors did not utilize guayule resin as a stabilizer as disclosed in this specification to prevent gel formation but rather to break down preformed gel.
Actually, Keller, Winkler and Stephens (Paper No. 55, "Degradative Effects of Guayule Resin on Natural Rubber", presented at the 117th Meeting of the Rubber Division, ACS; Las Vegas, Nev.; May 1980) reported the adverse effect of guayule resin on rubber. Specifically, these investigators reported that linoleic acid (which is a component of guayule resin) functioned as a prooxidant of guayule rubber. Further, the article entitled "Guayule, An Alternative Source of Natural Rubber" states on page 66 that during a milling operation some rubber may be degraded while it is in contact with guayule resin.
The preceding prior art citations would indicate that guayule resin would not have a stabilizing effect on rubber and possibly would be a relatively severe pro-oxidant. Unexpectedly, it has been discovered that guayule resin is an effective stabilizer for synthetic elastomers prepared in an emulsion or hydrocarbon system.